Passive force elements are used not only in motor vehicles. Since every moving object generates kinetic energy in a direction change or a stop, this energy must be taken up in a manner that is as controlled as possible. Known simple elements for taking up the kinetic energy are springs or rubber elements, however the stored energy must be dissipated elsewhere.
In the article xe2x80x9cSelection of Commercial Electrorheological Devicesxe2x80x9d, Proceedings of the Fourth International Conference on Electrorheological Fluids, 1994, pages 643 to 656, D. A. Brooks, FIG. 2 schematically shows a passive force element on the basis of electrorheological liquids. In this known force element, the kinetic energy of a moving object is transmitted via the piston rod with piston to the electrorheological liquid. The damping characteristic or the energy dissipation of the force element is regulated by an adjustable throttle opening, through which the electrorheological liquid flows during a shock impact. The adjustment of the throttle opening is achieved by applying a changeable voltage to the electrodes of a valve on the basis of electrorheological liquids, which operates in the squeeze mode.
Electrorheological liquids are liquids of which the Theological characteristics are controllable by the electrical field. Generally, electrorheological liquids are suspensions, i.e. solid particles suspended in a carrier medium, which are polarizable by the electrical field. Through the use of electrorheological liquids, it has become possible to embody actuators without moving parts or to considerably reduce the number of the moving parts.
Energy converters or transducers of electrorheological liquid actuators possess electrode arrangements, between which the electrorheological liquid is located, and onto which is applied the electrical control voltage. The interaction between the electrode arrangement and the electrorheological liquid can, dependent on the type of the liquid deformation, be distinguished among three basic modes, the shear mode (electrodes slide or shift relative to one another in parallel planes), the flow mode (electrodes are rigidly arranged, the liquid flows through between the electrodes) and the squeeze mode (electrodes vary their spacing distance relative to one another). These modes can also arise in combination. Further details in this regard are in the book: xe2x80x9cTechnischer Einsatz neuer Aktorenxe2x80x9d (xe2x80x9cTechnical Application of New Actuatorsxe2x80x9d), Expert-Verlag publisher, Renningen-Malmsheim, 1995, chapter 3.2.1 and FIG. 3.1.
In the above described known passive force element on the basis of electrorheological liquids, there arises a disadvantage due to the indirect control of the throttle opening by the valve on the basis of electrorheological liquids. Therefor, the regulation of the damping characteristic is only possible to a limited extent in this force element. During the shock impact process, undesired force peaks can act on the body that is to be braked or slowed down. Moreover, leakage losses of the force element cannot be compensated.
A main development goal of the automobile manufactures is the optimization of the cooperative interaction of vehicle occupants and restraint systems. The increased complexity of these systems, such as for example the airbag system and adaptive restraint systems, require the use of passive force elements, of which the damping characteristic is adjustable in a targeted and reproducible manner. Moreover, for example, a main characteristic or feature is the reduction of the maximum reaction forces/accelerations on the vehicle occupants.
Force elements are used in motor vehicle steering columns for limiting the impact force of a vehicle occupant onto the steering wheel or the airbag/steering wheel system. Such force elements absorb the kinetic energy of a shock impact, for example through the deformation or the destruction of components consisting of metal or plastic (rolled sheet metal arrangements, tearable or breakable strap). These force elements have the disadvantage that they cannot be used again after a shock impact has been exerted thereon. Moreover, a disadvantage exists therein, that they comprise a fixedly prescribed damping characteristic (force-path-characteristic curve).
Furthermore, from the EP 0 882 636, there is known a steering column with integrated force element, which consists of an outer and inner component. Under the influence of a longitudinal force exerted thereon, the components shift or slide relative to one another, whereby a damping material contained in the outer component becomes liquified, pulverized, or crystalized. In this known arrangement, the damping material must be exchanged after a shock impact has been exerted thereon, which is connected with a considerable effort. Moreover, only a determined force progression can be produced over the displacement path.
From the DE 197 49 970 A1, there is known an occupant safety apparatus for the driver""s side of a motor vehicle, in which an energy absorption element in the form of a piston cylinder arrangement is allocated to the steering spindle, which is guided in a shell or sleeve pipe. In the case of a crash, the piston cylinder arrangement of the steering column serves for the energy dissipation of an occupant impacting on the steering wheel. For adapting the damping characteristic to the severity of the accident, a different number of valves are actuated dependent on the mass and the relative velocity of the occupant.
It is an object of the present invention to further develop a force element in such a manner, so that a regulated damping characteristic can be adjustedly set, and force peaks are avoided.
This object is achieved in that the piston is coupled via an elastic element to the force introduction element, and thereby, in the first moment of a shock impact, an advantageous mass-spring-coupling is achieved, and therewith, force peaks at the beginning of the shock impact are avoided. In the further progression of the shock impact, that is to say after coupling-on of the piston, the piston is moved and the electrorheological liquid is displaced from the one working chamber into the other working chamber. Since the fluid connection connecting the working chambers comprises an electrorheological liquid valve, the damping characteristic can be adjusted by means of the height of the electrical field. In the use of the force element in or on the steering column of motor vehicles, an optimal damping characteristic can be adjustedly set dependent on sensor-measured occupant parameters, such as for example, the seating position, the size of the occupants, the weight of the occupants, the seat belt usage (yes or no) and/or the accident severity.
In connection with new discoveries or knowledge of biomechanics, or in connection with law changes, the force element already installed in motor vehicles can further continue to be used through simple adaptation of the control actuation.
By the embodiment or formation of a gas volume in a two-part piston, which consists of a lid part glidingly guided in a hollow cylindrical bottom part, and whereby the piston rod is glidingly supported in the lid part of the piston, only the small mass of the piston rod is accelerated at the beginning of the shock impact. The spring stiffness of the gas volume is very small at the beginning, and progressively varies in the further progression or course. Since the pressure of this gas volume is adjustable, the coupling conditions between piston rod and piston, and therewith the damping characteristic, can be adapted to the respective motor vehicle.
In a further embodiment it is provided, that the fluid connection connecting the working chambers is embodied by two concentrically arranged shells or sleeves forming the cylindrical housing, and the annular gap formed between the shells or sleeves is provided with controlledly energizable electrode surfaces for generating an electrical field. Through this embodiment, a direct regulation of the damping characteristic is possible at every point in time during the shock impact process.